Rethinking fish-friendliness of pumps by shifting focus to both safe and timely fish passage for effective conservation

Globally, catadromous freshwater eels of the genus Anguilla are of conservation concern, including critically endangered European eel (Anguilla anguilla). Pumping stations that move river water to a higher elevation severely impact eels during their seaward spawning migration. Fish-friendly pumps can mitigate fish injury and mortality but here we uniquely rethink a fish-friendly pump as a fish passage solution. In this pluriannual study, the seasonal timing of pump operation was misaligned with the typical silver eel migration period. Eels were almost exclusively nocturnal but night-time pumping represented as little as 5.6% a year. Night-time eel approaches were primarily influenced by pump duration and temperature, but did not align with lunar phase, unlike in unregulated rivers. After reaching the pumping station, eel passage was influenced by weedscreen aperture and increased when the aperture was increased. Passive sensor collision suggested non-pump infrastructure could cause injury and mortality to eels. It is therefore recommended pump operation should align with the timing of silver eel migration, weedscreen and pump entrance efficiencies should be maximised, and non-pump infrastructure must have low fish injury risk. Ultimately, considering the entire structure a fish passage solution will help ensure fish-friendly pumps have high conservation value for anguillid eels globally.


Supplementary tables and figures
Table S1 Alternative model selection (Generalised Linear Mixed Models; logit-linked binomial distribution) to determine passage probability of European eels which approached the pumping station.For each model the associated Akaiki Information Criteria (AIC) were calculated using glmmTMB in R 4.3.1.Light grey shading denotes model selected.S10.Metrics used for multi-beam sonar (ARIS) analysis of European eel approach, passage, and retreat behaviour.

Model
Table S11.Time series events used to analyse BDS data during pumping station passage.

Key event Definition
Injection Point when passage begins; pressure rise as pump chamber is entered

Table S2 .
Alternative model selection (Generalised Linear Mixed Models; logit-linked binomial distribution) to determine tactile behaviour probability of European eels which retreated from the pumping station.For each model the associated Akaiki Information Criteria (AIC) were calculated using glmmTMB in R 4.3.1.Light grey shading denotes model selected.

Table S3 .
Alternative model selection (Generalised Linear Model; log-linked Gamma distribution) to determine non-tactile response distance (m) of European eels which had a non-tactile retreat from the pumping station.For each model the associated Akaiki Information Criteria (AIC) were calculated using glmmTMB in R 4.3.1.Light grey shading denotes model selected.

Table S4 .
Results of Generalised Linear Mixed Models (GLMMs) examining the effects of pump duration, lunar phase and temperature on predicted count of night-time European eel approaches at the pumping station.GLMMs were fitted with log-linked zero-inflated negative binomial distributions and the random effect of month using package glmmTMB in R 4.3.1.

Table S5 .
Results of Generalised Linear Mixed Models (GLMMs) examining the effects of weedscreen aperture, eel approach speed and turbidity on passage probability of European eels which approached the pumping station.GLMMs were fitted with logit-linked binomial distributions and the random effect of year (mod3_b) using package glmmTMB in R 4.3.1.

Table S6 .
Results of Generalised Linear Mixed Models (GLMMs) examining the effects of weedscreen aperture, eel approach speed and turbidity on tactile behaviour probability of European eels which retreated from the pumping station.GLMMs were fitted with logit-linked binomial distributions and the random effect of year (mod2_b) using package glmmTMB in R 4.3.1.

Table S7 .
Results of Generalised Linear Models (GLMs) examining the effects of weedscreen aperture, eel approach speed and turbidity on non-tactile response distance (m) of European eels which had a non-tactile retreat from the pumping station.GLMs were fitted with log-linked Gamma distributions using package glmmTMB in R 4.3.1.

Table S8 .
Observed percentage passage (n passed / n retreated) for European eel that approached the 100-mm, 212-mm (1-m section) and 190-mm aperture weedscreen in each year and in total.denoteswere there are no data.

Table S9 .
Observed percentage tactile response (n tactile / n non-tactile) for European eel that retreated from the 100-mm, 212-mm (1-m section) and 190-mm aperture weedscreens in each year and in total.denoteswhere there ae no data.